The present invention relates to a method of controlling power in a transmission link between a central station and a terminal in a point to multi-point communication network and system for carrying out said method. The present invention is suitable for implementation in satellite access networks or terrestrial point to multi-point access networks.
Particularly the invention relates to controlling power in communication between a transmitter such as a central station and terminals receiving such communication, which permit the compensation of power loss caused by the diverse phenomena present on the way of the signal propagation.
As it is well known in the field of telecommunications, when a signal is transmitted from a transmitter station via radio to a receiver station, different phenomena present on its propagation path may affect the quality of the signal received at the receiver end.
One example of such phenomena may be obstacles present on the propagation path of the signal which cause the signal to be reflected from them and thus give rise to signals traveling on various paths, or the so called multi-path signals. An important negative consequence caused by the presence of multi-path signals is the effect known as fading in which signals transmitted through different paths are received in different phases and therefore tend to cancel each other in amplitude.
Rain, storm or other similar natural disturbances are further examples of phenomena giving rise to signal deterioration. A further effect in power loss variation could be caused by the distance variation from the base station, or from the terminal to the satellite when the terminal and/or the satellite are moving. It is known that the larger said distance is, the higher the power loss would be in the signal. Overcoming these effects in conventional manners would require costly solutions and limits the resources of the whole system. A widely used known solution consists of adjusting the power, link by link, or connection by connection. The terminal station performs in a continuous manner a measurement of the quality of the data received. Said measurement is then communicated to the central station. The central station would then adjust the power of transmission depending upon the information received from the terminal.
The above solution has the drawback that when traffic links transmit the data in a sporadic mannerxe2x80x94which is a situation which may occur for example in Internet traffic, voice transmission using silence intervals, or the likexe2x80x94it may happen that a traffic link remains inactive during a relatively long period of time due to the presence of the aforementioned transmission phenomena, thus making said measurement impossible or inaccurate. A solution to this problem may consist of forcing a cyclic emission in the central station during said silence periods; however this approach would cause loss of resources in the transmission system.
According to another known solution the terminal performs a quality calibration over the signal broadcast from the central station. An example of calibration could consist of measuring the bit error rate of the signal to noise ratio. However if the dynamic of the power control is important, it could turn out to be difficult, or even impossible to determine with good accuracy, the quality of the received signal when the its level is too high. This situation may occur when a reference signal is emitted at strong levels so that it can be received by all the terminals, including under unfavorable propagation conditions.
Therefore it is desired to compensate the effects caused by the phenomena present on the transmission path of the signal, while at the same time the use of the resources of the transmission system is optimized. In order to obtain the desired solution it would be necessary to control the transmission power.
In order to overcome the above disadvantages, the method of controlling power in a transmission link between a transmitter and a receiver in a point to multi-point communication network and system for carrying out said method are proposed by the present invention.
One object of the present invention is that of providing a method comprising the steps of:
at least one transmitter means periodically transmitting a plurality of reference signals to a plurality of receiver means; each reference signal having a quality corresponding to at least one receiver means;
each receiver means receiving said plurality of reference signals and measuring their respective quality so as to determine the reference signal corresponding to said receiver means;
each receiver means indicating to said at least one transmitter means, the reference signal corresponding thereto; and if the indication received from said receiver means so requires,
said at least one transmitter means adjusting the power level of transmission.
According to the invention, said quality of each reference signal is expressed in terms of signal to noise ratio or other known means.
The indication of the reference signal corresponding to the receiver means is made by transmitting a return signal from the receiver means to the transmitter means.
Further, in order to increase the accuracy and to limit the number of transmitted signals, the return signal indicated by the receiver means to the transmitter means ma be computed from the received reference signals.
Another object of the present invention is that of providing a transmitter means adapted for periodically transmitting a plurality of reference signals to a plurality of receiver means, each reference signal having a transmission quality corresponding to at least one receiver means; and for receiving signals transmitted from at least one receiver means, said signals being indicative of a reference signal corresponding to said receiver means.
The above and further embodiments of the present invention are described in further detail hereinbelow as well as in the claims.